The transmission capability of a frictionally engaged shift element, for example a multi-disk clutch, multi-disk brake, etc., of a transmission device of a vehicle drive train, in particular a multi-stage automatic transmission or a transverse transfer case, or an all-wheel distributor clutch, can normally be varied by applying an actuation force between a minimum at which the frictionally engaged shift element is completely disengaged and the transmission capability is substantially zero, and a maximum at which the frictionally engaged shift element is substantially fully engaged and is in a slip-free operating state.
In order to change the operating state of the frictionally engaged shift element requested by a higher-level driving strategy in which the frictionally engaged shift element is to be inserted in a flow of force of a vehicle drive train while maintaining a high level of driving comfort, it is necessary to know the precise applied actuation force of the frictionally engaged shift element to be inserted at which the transmission capability of the shift element is substantially zero, and starting at which an increase in the actuating force increases the transmission capability of the shift element. The halves of the frictionally engaged shift element lie against each other, or are in contact with each other, while the actuation force is being applied, without any significant torque being transmitted from one shift element half to the other shift element half, wherein such an operating state of a frictionally engaged shift element is also referred to as the touchpoint. If shift elements of transmission devices are hydraulically actuated, the applied actuation force is provided by a specific applied actuation pressure value which changes over the service life.
Frictionally engaged shift elements which are hydraulically actuated differential locks of transverse transfer cases or all-wheel distributor clutches are operated in a state of permanent slip, as is known. In addition to a robust design, particularly high demands are placed on the actuating precision, actuating dynamics and disengagement dynamics of such frictionally engaged shift elements to provide the optimum driving dynamics for the current driving situation. However, these requirements can be implemented only to the desired extent when the touchpoint of a frictionally engaged shift element is known.
In transmission devices known from practical applications, the touchpoint of frictionally engaged shift elements is determined cyclically over the service life of a frictionally engaged shift element, wherein the applied actuation pressure value is determined by means of rotational speed sensors or torque sensors located in the region of the shift element halves. When sensors are arranged at a distance from the shift element, sophisticated measuring equipment must be used to obtain sufficiently accurate measured values, due to component tolerances.
In addition, sensors arranged in the region of rotating components require construction space and increase the manufacturing cost of a transmission device. Furthermore, the actuation pressure value cannot be adapted depending on the service life when there is a malfunction in the region of the sensors, whereby the servicing of a transmission device is increased to an undesired extent.